1. Field of the Invention
The subject invention relates generally to an electromagnetically actuated valve assembly, and more particularly toward a canister purge valve assembly of the type used for controlling the flow of fuel vapors from a fuel tank to an internal combustion engine.
2. Related Art
A canister purge valve (CPV) is an electromagnetically driven valve that allows fuel vapors that are collected from the fuel tank in a canister to be purged by suction through an engine intake manifold under certain operating modes and conditions. A typical prior art CPV is illustrated in FIG. 1. As shown, the construction includes an inlet tube that leads to a chamber. An outlet tube has an upstanding portion that resides in the chamber and presents a generally annular valve seat for contact by a poppet valve having a flat valve bottom surface to seal the valve seat when in a closed condition. The poppet valve is controlled by an electromagnetic actuator which is schematically depicted in the traditional manner in FIG. 1.
When the valve operates under the timed influence of the electromagnetic actuator, it oscillates quite rapidly, typically in the frequency range of 5-20 Hertz. When the valve operates in this way, two types of noises are observed. One is a ticking noise caused by the opening and closing of the valve. This ticking noise is transmitted into the passenger compartment via hard mounting of the vapor lines. Particularly, the ticking noise is found to occur if the vapor line mounting is made at the front of the floor pan. The other type of noise is a whooshing noise that is caused by the flow of gases through the valve as they are sucked into the engine intake manifold.
The whooshing sound is considered to be particularly troublesome. Noise, vibration and harshness (NVH) concerns during vehicle operation can greatly affect customer satisfaction with the vehicle. Vehicle purchase decisions can be influenced by NVH characteristics. Accordingly, all stray and undesirable noises are a matter of particular concern to vehicle designers and manufacturers.
The current hypothesis is that a shockwave present in the flow of fuel vapors, which passes through the valve, expands into either the vapor line (for in-line mounting) or into the manifold (for direct mounting). Some vehicles have better under hood insulation that prevents this noise from being heard with the hood down or while inside the passenger compartment. A typical complaint, however, is that this predominant whooshing noise is heard when the vehicle hood is open and a person is standing near the engine.
An objectionable noise also occurs during the closing event of the valve, which creates a popping noise that reverberates through the vapor lines. This popping noise phenomenon happens during normal operation, where the engine intake manifold pulls a vacuum on the outlet tube side of the valve. Mechanical analysis, including flow analysis and testing, have demonstrated that failures are caused by flow-induced instability in the valve. Particularly, during the conversion of the momentum energy into potential energy in the gas flow below the flat bottom of the valve and the narrow gap between the valve and the valve seat, a stagnation of flow and separation occurs. The stagnation and separation zones under the valve are significantly unstable and result in local pressure pulsations and noise. These phenomenon are graphically illustrated in FIGS. 2 and 3. During the energy conversion process, i.e., momentum to potential energy, energy dissipation contributes to the aerodynamic noise and mechanical valve vibrations.
Although the prior art has recognized the symptom of objectionable noise, they have taken diverse steps to evade the problem. Primary solutions found in the prior art include steps of isolating the sound emissions from the acoustic source to the intake manifold. For example, there have been proposals to avoid hard mounting of vapor lines to the passenger compartment as a means of isolating flow noises. This is not a solution aimed at preventing the noise from occurring, but merely a method to mask or minimize vibrations which transmit and amplify the noise. Another prior art solution attempts to absorb the sound energy by inserting foam or other acoustic absorbing material at a downstream outlet from the valve. Such proposals are somewhat effective, but do not eliminate the problem and add costs to the overall system.
Likewise, dampers can be added for reducing the probability of sound column resonance. Such muffler and sound dampening strategies are not well suited to practical, real world applications. Furthermore, cost and design complexity issues severely frustrate such approaches.